Biosensors are devices for sensing and detecting biomolecules and operate on the basis of electronic, electrochemical, optical, and/or mechanical detection principles. Biosensors that include transistors are sensors that electrically sense charges, photons, and/or mechanical properties of bio-entities or biomolecules. The detection can be performed by detecting the bio-entities or biomolecules themselves, or through interaction and reaction between specified reactants and bio-entities/biomolecules. Such biosensors can be manufactured using semiconductor processes and can be easily applied to integrated circuits (ICs) and microelectromechanical systems (MEMS).